The invention relates to a method of compensating the interference DC voltages in the electrode circuit in magnetic-conductive flow measurement with periodically reversed DC magnetic field in which the useful signal is obtained by sampling and storing the signal voltage after each reversal of the magnetic field at opposite polarity values of said magnetic field during a sampling time interval and forming the difference of the stored sampled values, and wherein in a compensating time interval following each sampling time interval a compensation voltage is produced by sampling and storing the signal voltage, which compensation voltage is superimposed oppositely on the signal voltage for compensating the signal voltage within the compensation time interval to the value zero and is retained until the next compensation time interval.
A method of this type is known from U.S. Pat. No. 4,210,022. The difference of the sampling values obtained with opposite polarity values of the magnetic field gives a useful signal which is freed from constant interference DC voltages which as is known in the magnetic-inductive flow measurement can be as much as a thousand times greater than the useful signal. By the formation and superimposing of an additional compensation voltage linear time variations of the interference DC voltages between the successive sampling time intervals are also compensated and above all the amplifiers and subtraction circuits used to process the signal voltage are prevented from being overdriven due to the interference DC voltages building up slowly to a very high value.
In this known method each compensation time interval lies in a pause in the magnetic field which is inserted in each case between two successive partial periods in which the magnetic field assumes its oppositely poled value. The signal voltage sampled to form the compensation voltage is thus the pure interference voltage. The magnetic field must therefore be periodically switched between three values such that the pauses in the magnetic field and the compensation time intervals contained therein cannot be utilized for observing the flow.
In similar manner, in a method known from U.S. Pat. No. 4,010,644 a compensation of time variations of the interference DC voltages is carried out by forming a compensation voltage which is oppositely superimposed on the signal voltage. In this known method the magnetic field is switched to and fro between two different values, one of which may be zero. In this case in each partial period in which the magnetic field has the smaller value (or the value zero) two compensation time intervals lie at the start and the end of the partial period respectively so that the compensation voltage stored at the end of the partial period and effective during the next sampling time interval is again the pure interference voltage. In this case the entire magnetic field partial period which contains the compensation time intervals is not available for observation of the flow. Compared with the previously outlined known method there is the further disadvantage that for the same power necessary for producing the magnetic field the measured value signal obtained is only half as great and consequently the signal-to-noise ratio is poorer.
The problem underlying the invention is the provision of a method of compensating interference DC voltages in the magnetic-conductive flow measurement which permits the maximum possible utilization of the available time for observing the flow and provides an increased useful signal for the same power expenditure.
According to the invention this is achieved in that each compensation time interval lies within the time interval, corresponding to the switched-on magnetic field, in which also the preceding sampling time interval lies.
With the method according to the invention, contrary to the known methods, the compensation voltage formed in each compensation time interval is not the pure interference voltage but contains the same useful voltage as the sampling value obtained and stored in the preceding sampling time interval. Since this compensation voltage in the following sampling time interval remains superimposed on the signal voltage in the opposite sense, for forming the stored sampling value a voltage is sampled which corresponds to the sum of the useful voltages of two successive magnetic field partial periods. In this manner the compensation time interval is also used for obtaining the measured value signal and observing the flow. Furthermore, the method is also suitable when the magnetic field is switched to and fro without pauses between two oppositely poled values so that no magnetic field intervals are lost for the observation of the flow.
Further features and advantages of the invention will be apparent from the following description of an example of embodiment with the aid of the drawings, wherein: